Carbamates are valuable intermediates in the production of agrochemicals, dyes, pharmaceutical compounds and, in particular, aromatic isocyanates used in the synthesis of polyurethanes. Most relevant from a commercial point of view are carbamates derived from 4,4′-methylenediphenylamine (MDA), its isomers and/or homologues or mixtures of the aforementioned compounds as obtained by acid catalyzed condensation/rearrangement reaction of aniline and formaldehyde, as well as 2,4-toluenediamine (TDA) or technical mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA (approximately 80/20 mixtures). The aforementioned aromatic amines are used in the preparation of methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), which are the direct precursors of polyurethanes. At present these isocyanates are produced industrially by phosgenation of the corresponding amines, a process which employs a toxic reagent (phosgene) and leads to large amounts of hydrochloric acid as side-product.
In the prior art, processes are known for the production of carbamates based on the functionalization of aromatic amines Ar—NH2 with organic carbonates ROCO2R in the presence of suitable catalysts, according to the following scheme:

In the case of aromatic diamines Ar(—NH2)2, biscarbamates are formed in a two-step reaction, with the corresponding monocarbamates being formed as intermediates, according to the following scheme:

Taking into account the alkylating properties of organic carbonates, N-alkylation competes with N-alkoxycarbonylation, and consequently N-alkylated products might be formed along the reaction, as well as products which are both N-alkylated and N-alkoxycarbonylated.
In the U.S. Pat. No. 3,763,217 it is disclosed that the Lewis acids SbCl5, SbCl3, SbCl2, AlCl3, SbF3, FeCl3, UO2(NO3)2, UO2, UO3, NbCl5 and ThCl4 are suitable catalysts for reacting an organic carbonate with an aromatic amine to prepare carbamates. In the U.S. Pat. No. 4,268,683 and the European patent application EP-A-0065026 a process is disclosed for preparing carbamates from organic carbonates and aromatic amines in the presence of catalytic quantities of a Lewis acid catalyst. The catalyst should be soluble in the reaction mixture at the reaction conditions and be a member of the group consisting of a zinc or divalent tin halide, a zinc or divalent tin salt of a monovalent organic compound which has a pKa value of at least 2.8, and a zinc or divalent tin salt of trifluoroacetic acid. Among the zinc salts are mentioned: zinc chloride, zinc acetate, zinc acetate dihydrate, zinc oxyacetate ((AcOZn)2O), zinc naphthenate, zinc octanoate, zinc propionate, zinc salicylate, zinc pivalate, zinc acrylate, zinc p-chlorobenzoate, zinc phenolate, zinc formate, zinc chloroacetate, zinc acetylacetonate, zinc oxalate, and zinc trifluoroacetate.
In the article of Baba et al., “Catalytic Synthesis of Dimethyl toluene-2,4-dicarbamate by the Methoxycarbonylation of 2,4-Toluenediamine with Dimethyl Carbonate Using Zn(OAc)2.H2O”, Science and Technology in Catalysis, 2002, 149, the reaction of the amines MDA and TDA with dimethyl carbonate is described in the presence of a metal salt as catalyst to obtain the corresponding dicarbamates. Several salts of zinc, tin, lead and bismuth are mentioned. It is also disclosed that the selection of the metal salt is crucial for the formation of the carbamates. Among the catalysts some zinc carboxylates showed catalytic activity, while others were inactive. For instance, in the reaction of TDA with dimethyl carbonate, zinc acetate dihydrate as catalyst yielded 92% of dicarbamate, whereas zinc propionate yielded only 20% and zinc formate was completely inactive.
Another article of Baba et al., “Catalytic methoxycarbonylation of aromatic diamines with dimethyl carbonate to their dicarbamates using zinc acetate”, Catalysis Letters, 2002, 82, 193-197, discloses the preparation of dicarbamates by methoxycarbonylation of TDA and MDA with dimethyl carbonate using zinc acetate dihydrate, or anhydrous zinc acetate as catalysts. The yield in the methoxycarbonylation of TDA with dimethyl carbonate using the hydrated catalyst is 92%, and using the anhydrous catalyst it is 98%. In the case of MDA the yield with zinc acetate dihydrate as catalyst is 98%.
Finally, in the article “Characteristics of methoxycarbonylation of aromatic diamine with dimethyl carbonate to dicarbamate using zinc acetate as catalyst”, Green Chemistry, 2005, 7, 159-165, Baba et al. describe the reaction of the aromatic amines TDA and m-phenylenediamine with dimethyl carbonate in the presence of zinc acetate dihydrate as catalyst.
In the article “Zinc Acetates as Efficient Catalysts for the Synthesis of Bis-isocyanate Precursors”, Industrial and Engineering Chemistry Research, 2010, 49, 6362-6366, Reixach et al. describe the use of zinc acetate dihydrate and anhydrous zinc acetate in alkoxycarbonylation of TDA and MDA using dimethylcarbonate and diethylcarbonate.
EP-A-1268409 describes the usage of zinc acetate dihydrate as catalyst in a continuous process for the manufacturing of aromatic carbamates by reaction of 80/20 mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with dimethyl carbonate. In EP-A-1255728, Zn salts such as zinc acetate or zinc acetate dihydrate (amongst other compounds) are mentioned as catalysts for the synthesis of aromatic carbamates by reaction of aromatic amines like 80/20 mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with dimethyl carbonate. Compounds or salts of Sn, Zn or Pb in particular are described as catalysts for the reaction of 2,4-TDA or technical mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with diethyl carbonate in EP-A-520273, or for the reaction of MDA (that is 4,4′-MDA, its isomers and/or homologues or mixtures of the aforementioned compounds as obtained by acid catalyzed condensation/rearrangement reaction of aniline and formaldehyde) with dialkyl carbonates like dimethyl carbonate or diethyl carbonate in EP-A-510459. In the European patent application EP-A-1958940, the inventors disclose processes for preparing azolynes, cyanoazolynes, symmetrical and unsymmetrical bisazolynes, amides, bisamides, cyanoamides, and peptides, which comprise the use of a metal catalyst defined by the general formula Zna(OCOR7)bZ2c, wherein R7 represents an optionally substituted alkyl group or an optionally substituted aryl group; Z2 represents an oxygen atom, a sulfur atom, or a selenium atom, “a” represents 1 or 4, “b” represents 2 or 6, and “c” represents 0 or 1; and provided that when “a” is 1, “b” is 2 and “c” is 0, and when “a” is 4, “b” is 6 and “c” is 1. The following zinc salts are specified in that patent application: zinc acetate, zinc trifluoroacetate, zinc acetoacetonate, zinc acetylacetonate, zinc trifluomethanesulfonate, and zinc p-toluenesulfonate. Furthermore, it is disclosed that certain tetranuclear zinc clusters may be used as catalysts, for instance: Zn4(OAc)6O, Zn4(OCOEt)6O, Zn4(OPv)6O, Zn4[OCO(CH2)16CH3]6O, Zn4(OCOPh)6O and Zn4(OCOCF3)6O, wherein Ac represents an acetyl group, Et represents an ethyl group, Pv represents a pivaloyl group, and Ph represents a phenyl group. The zinc cluster Zn4(OAc)6O is used in the preparation of oxazolynes and peptides.
EP 2 230 228 A1 and WO 2010/105768 A1 disclose a process for preparing aromatic carbamates using the tetranuclear zinc cluster Zn4(OAc)6O as catalyst. In the article “Alkoxycarbonylation of Industrially Relevant Anilines Using Zn4O(O2CCH3)6 as Catalyst”, Industrial and Engineering Chemistry Research, 2012, 51, 16165-16170, Reixach et al. describe the use of Zn4(OAc)6O as catalyst for methoxy- and ethoxycarbonylation of a wide variety of amines, including 13 examples of aromatic amines.
Other contributions in this technical field include the reaction of aniline with dimethyl carbonate in the presence of homogeneous, heterogeneous and heterogenized (silica- or alumina-supported) zinc catalysts (Grego et al., Pure and Applied Chemistry, 2012, 84, 695-705). Furthermore, ionic liquid-promoted zinc acetate catalysts (Zn(OAc)2-ILs) have been reported to mediate the same reaction (Zhao et al., Industrial and Engineering Chemistry Research, 2012, 51, 11335-11340).